The present invention relates to a method by which various liquid compositions are curtain-coated onto a continuously running support in strip form (which is hereinafter referred to as a "web") in the manufacture of photographic films, photographic papers, magnetic recording tapes, adhesive tapes, pressure-sensitive recording papers, offset printing plates, etc.
The basic technology of curtain coating is described in U.S. Pat. Nos. 3,508,947 and 3,632,374. In "AIChE Winter National Meeting" (1982), S. F. Kistler disclosed the theory of curtain coating, focusing on the following three phenomena which he considered would govern the rate of application by curtain coating:
(1) incorporation of tiny air bubbles between the web and the coating solution (which phenomenon is hereinafter referred to as "air entrainment"); PA1 (2) formation of a liquid deposit along the line where the coating solution contacts the web (which phenomenon, hereinafter referred to as "heel", is common with large coating weights); and PA1 (3) the coating solution is not adequately deposited but will bounce off the web being coated (which phenomenon, hereinafter referred to as "sagging", is caused by "heel with air entrainment" and is also common with large coating weights ). PA1 (1) replacing the web-entrained air layer with carbon dioxide to suppress the phenomenon of "air entrainment" (see U.S. Pat. No. 4,842,900); PA1 (2) applying a static electric field between the web and the coating solution, whereby the adhesion of the latter is enhanced to suppress the phenomenon of "air entrainment" (see Unexamined Published Japanese Patent Application No. 197176/1987); and PA1 (3) stabilizing the deposition of the free falling curtain on the web by specifying the shape of the tip of the sliding surface and the angle at which the free falling curtain is deposited on the web (see Unexamined Published Japanese Patent Application No. 51170/1989). PA1 (4) suppressing the phenomenon of "heel" by properly adjusting the viscosity between the lower and upper layers of coating solution (see Unexamined Published Japanese Patent Application No. 131549/1989).
According to Kistler, curtain coating is no longer possible if one or more of these phenomena occur.
Various attempts have been made to increase the curtain coating speed limited by the aforementioned phenomena. They include:
In fact, however, as modern coating plants adopt application speeds of 250 m/min and higher with the curtain of coating solution flowing down in higher rates, the limitation of coating speeds by "heel" and "sagging" has become a greater concern than the limitation by "air entrainment". A method that has been proposed for dealing with this problem is:
The techniques described in Unexamined Published Japanese Patent Application Nos. 51170/1989 and 131549/1989 are such that the flow rate of coating solution is in the range of 1.0-4.0 cm.sup.3 /cm.sec (the unit length of the coating width being expressed in centimeters). These techniques are effective at flow rates within the specified range, but no study has been conducted to determine whether they are effective in suppressing the phenomenon of "sagging" in flow quantities exceeding 4 cm.sup.3 /cm.sec.